GNU bug report logs - #78082
31.0.50; Proposal for this-command display buffer action alist key

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Sean Whitton <spwhitton <at> spwhitton.name>
To: bug-gnu-emacs <at> gnu.org
Subject: 31.0.50; Proposal for this-command display buffer action alist key
Date: Sun, 27 Apr 2025 16:46:45 +0800

X-debbugs-cc: juri <at> linkov.net

I want windows generated by C-x v = to be displayed but not selected,
but windows generated by C-x v D to be both displayed and selected.

I have had advice in my init.el for a long time, using
save-selected-window, to achieve this.  But I just learned about the
post-command-select-window display buffer action alist key, which would
be a better solution.

It's not quite applicable, however, due to how C-x v = and C-x v D both
use a buffer called *vc-diff*.  Therefore, I can't distinguish them in
an entry in display-buffer-alist, I think?

I would like to propose a new display buffer action alist key called
this-command.  Its value would be a command or list of commands.
The display-buffer-alist entry would apply only when the 'this-command'
variable matched the 'this-command' display buffer action alist entry.
I could then use

    (("^\\*vc-diff\\*"
      nil
      (this-command . (vc-diff log-view-diff log-view-diff-changeset))
      (post-command-select-window . t))
     ("^\\*vc-diff\\*"
      nil
      (this-command . vc-root-diff)
      (post-command-select-window . nil)))

Juri, what do you think?

-- 
Sean Whitton

Message #8 received at 78082 <at> debbugs.gnu.org (full text, mbox):

From: Juri Linkov <juri <at> linkov.net>
To: Sean Whitton <spwhitton <at> spwhitton.name>
Cc: 78082 <at> debbugs.gnu.org
Subject: Re: bug#78082: 31.0.50; Proposal for this-command display buffer
 action alist key
Date: Mon, 28 Apr 2025 18:09:35 +0300

> I want windows generated by C-x v = to be displayed but not selected,
> but windows generated by C-x v D to be both displayed and selected.
>
> I have had advice in my init.el for a long time, using
> save-selected-window, to achieve this.  But I just learned about the
> post-command-select-window display buffer action alist key, which would
> be a better solution.
>
> It's not quite applicable, however, due to how C-x v = and C-x v D both
> use a buffer called *vc-diff*.  Therefore, I can't distinguish them in
> an entry in display-buffer-alist, I think?
>
> I would like to propose a new display buffer action alist key called
> this-command.  Its value would be a command or list of commands.
> The display-buffer-alist entry would apply only when the 'this-command'
> variable matched the 'this-command' display buffer action alist entry.
> I could then use
>
>     (("^\\*vc-diff\\*"
>       nil
>       (this-command . (vc-diff log-view-diff log-view-diff-changeset))
>       (post-command-select-window . t))
>      ("^\\*vc-diff\\*"
>       nil
>       (this-command . vc-root-diff)
>       (post-command-select-window . nil)))
>
> Juri, what do you think?

Currently the official way to do this is to use a function predicate
in the condition part of the display-buffer-alist rule like this:

(add-to-list 'display-buffer-alist
             `(,(lambda (buffer-name _action)
		 (and (memq this-command '( vc-diff log-view-diff
					    log-view-diff-changeset))
		      (string-match-p "^\\*vc-diff\\*" buffer-name)))
               nil
               (post-command-select-window . t)))

(add-to-list 'display-buffer-alist
             `(,(lambda (buffer-name _action)
		 (and (eq this-command 'vc-root-diff)
		      (string-match-p "^\\*vc-diff\\*" buffer-name)))
               nil
               (post-command-select-window . nil)))

To simplify this, a special 'this-command' item could be supported as well,
but it should be in the condition part e.g.

(add-to-list 'display-buffer-alist
             '((this-command . vc-root-diff)
               nil
               (post-command-select-window . nil)))

like there is a 'category' item:

(add-to-list 'display-buffer-alist
             '((category . diff)
               nil
               (post-command-select-window . nil)))

But then the corresponding predicate for 'this-command'
should be added to 'buffer-match-p' when deemed necessary.

Message #11 received at 78082 <at> debbugs.gnu.org (full text, mbox):

From: Sean Whitton <spwhitton <at> spwhitton.name>
To: Juri Linkov <juri <at> linkov.net>
Cc: 78082 <at> debbugs.gnu.org
Subject: Re: bug#78082: 31.0.50; Proposal for this-command display buffer
 action alist key
Date: Tue, 29 Apr 2025 08:32:15 +0800

Hello,

On Mon 28 Apr 2025 at 06:09pm +03, Juri Linkov wrote:

>> I want windows generated by C-x v = to be displayed but not selected,
>> but windows generated by C-x v D to be both displayed and selected.
>>
>> I have had advice in my init.el for a long time, using
>> save-selected-window, to achieve this.  But I just learned about the
>> post-command-select-window display buffer action alist key, which would
>> be a better solution.
>>
>> It's not quite applicable, however, due to how C-x v = and C-x v D both
>> use a buffer called *vc-diff*.  Therefore, I can't distinguish them in
>> an entry in display-buffer-alist, I think?
>>
>> I would like to propose a new display buffer action alist key called
>> this-command.  Its value would be a command or list of commands.
>> The display-buffer-alist entry would apply only when the 'this-command'
>> variable matched the 'this-command' display buffer action alist entry.
>> I could then use
>>
>>     (("^\\*vc-diff\\*"
>>       nil
>>       (this-command . (vc-diff log-view-diff log-view-diff-changeset))
>>       (post-command-select-window . t))
>>      ("^\\*vc-diff\\*"
>>       nil
>>       (this-command . vc-root-diff)
>>       (post-command-select-window . nil)))
>>
>> Juri, what do you think?
>
> Currently the official way to do this is to use a function predicate
> in the condition part of the display-buffer-alist rule like this:
>
> (add-to-list 'display-buffer-alist
>              `(,(lambda (buffer-name _action)
> 		 (and (memq this-command '( vc-diff log-view-diff
> 					    log-view-diff-changeset))
> 		      (string-match-p "^\\*vc-diff\\*" buffer-name)))
>                nil
>                (post-command-select-window . t)))
>
> (add-to-list 'display-buffer-alist
>              `(,(lambda (buffer-name _action)
> 		 (and (eq this-command 'vc-root-diff)
> 		      (string-match-p "^\\*vc-diff\\*" buffer-name)))
>                nil
>                (post-command-select-window . nil)))
>
> To simplify this, a special 'this-command' item could be supported as well,
> but it should be in the condition part e.g.
>
> (add-to-list 'display-buffer-alist
>              '((this-command . vc-root-diff)
>                nil
>                (post-command-select-window . nil)))
>
> like there is a 'category' item:
>
> (add-to-list 'display-buffer-alist
>              '((category . diff)
>                nil
>                (post-command-select-window . nil)))
>
> But then the corresponding predicate for 'this-command'
> should be added to 'buffer-match-p' when deemed necessary.

After reading your message, I think this would be even more useful than
just for solving my case.

When people match on buffer names, they are usually just matching on all
the buffers produced by a given command or commands.  So it might make
for more declarative, readable configuration if they can just use
this-command.

Thank you for the hints about the condition part and buffer-match-p.
I'll implement it.

-- 
Sean Whitton

Message #16 received at 78082-done <at> debbugs.gnu.org (full text, mbox):

From: Sean Whitton <spwhitton <at> spwhitton.name>
To: Juri Linkov <juri <at> linkov.net>, 78082-done <at> debbugs.gnu.org
Subject: Re: bug#78082: 31.0.50; Proposal for this-command display buffer
 action alist key
Date: Thu, 01 May 2025 20:59:33 +0800

Hello,

Implemented and installed this.

-- 
Sean Whitton

Message #19 received at 78082 <at> debbugs.gnu.org (full text, mbox):

From: Juri Linkov <juri <at> linkov.net>
To: Sean Whitton <spwhitton <at> spwhitton.name>
Cc: 78082 <at> debbugs.gnu.org
Subject: Re: bug#78082: 31.0.50; Proposal for this-command display buffer
 action alist key
Date: Thu, 01 May 2025 20:41:48 +0300

> Implemented and installed this.

Thanks.  I noticed one problem in the documentation:

'this-command' is not a buffer display action alist entry, but
a buffer display condition entry.  I'm sorry that this confusion
was caused by the 'category' entry used for both condition and action:
a display-buffer call can provide the action entry 'category',
then the 'category' condition can match it in 'display-buffer-alist'.

So we need to move the description of 'this-command' from
(info "(elisp) Buffer Display Action Alists") to
to (info "(elisp) Buffer List") where it could follow
the 'category'.

Also 'this-command' could be mentioned in
(info "(elisp) Choosing Window") after 'category' as well.

'this-command' is correctly documented in the docstring of
'buffer-match-p', but should not be presented in the docstring
of 'display-buffer' (maybe only as an example?)

Message #22 received at 78082 <at> debbugs.gnu.org (full text, mbox):

From: Sean Whitton <spwhitton <at> spwhitton.name>
To: Juri Linkov <juri <at> linkov.net>
Cc: 78082 <at> debbugs.gnu.org
Subject: Re: bug#78082: 31.0.50; Proposal for this-command display buffer
 action alist key
Date: Fri, 02 May 2025 12:49:58 +0800

Hello,

On Thu 01 May 2025 at 08:41pm +03, Juri Linkov wrote:

>> Implemented and installed this.
>
> Thanks.  I noticed one problem in the documentation:
>
> 'this-command' is not a buffer display action alist entry, but
> a buffer display condition entry.  I'm sorry that this confusion
> was caused by the 'category' entry used for both condition and action:
> a display-buffer call can provide the action entry 'category',
> then the 'category' condition can match it in 'display-buffer-alist'.

Ah, I see.  Thanks.

> So we need to move the description of 'this-command' from
> (info "(elisp) Buffer Display Action Alists") to
> to (info "(elisp) Buffer List") where it could follow
> the 'category'.

Done.

> Also 'this-command' could be mentioned in
> (info "(elisp) Choosing Window") after 'category' as well.

Yes, added something there.

> 'this-command' is correctly documented in the docstring of
> 'buffer-match-p', but should not be presented in the docstring
> of 'display-buffer' (maybe only as an example?)

display-buffer's docstring doesn't have worked examples in it so adding
one for this doesn't seem right.
I think we have to rely on (info "(elisp) Choosing Window").

-- 
Sean Whitton

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